Exhaust gas purification device for internal combustion engine

ABSTRACT

An auxiliary agent supply means ( 32, 58 ) is provided to supply, upstream of an exhaust gas purification means ( 24, 56, 78 ), an auxiliary agent for maintaining the exhaust gas purifying function of the exhaust gas purification means ( 24, 56, 78 ). A control means ( 38 ) for controlling the auxiliary agent supply means ( 32, 58 ), thereby regulating the amount of the auxiliary agent supplied includes a standard supply quantity determination section ( 40, 50, 68, 86 ) for determining standard supply quantity of the auxiliary agent required to maintain the exhaust gas purifying function, a target supply quantity determination section ( 42, 52, 70, 88 ) for determining target supply quantity of the auxiliary agent, by correcting the standard supply quantity on the basis of exhaust pressure, and a supply control section ( 44, 54, 72, 90 ) for controlling the auxiliary agent supply means to supply the auxiliary agent in the target supply quantity.

TECHNICAL FIELD

The present invention relates to an exhaust gas purification device for an internal combustion engine, and more specifically, an exhaust gas purification device using an auxiliary agent to maintain an exhaust gas purifying function.

BACKGROUND ART

In order to convert pollutants contained in the exhaust gas of an internal combustion engine, such as HC (carbon hydrate), CO (carbon monoxide) and NOx (nitrogen oxides), to purify the exhaust gas, an exhaust gas purification catalyst is conventionally used. In the case of a diesel engine, a particulate filter for trapping particulate matter contained in exhaust is used, in addition to such exhaust gas purification catalyst. In some of exhaust gas purification devices such as exhaust gas purification catalysts and particulate filters, an auxiliary agent is used to maintain their exhaust gas purifying function.

In some particulate filters, when the particulate matter trapped and accumulated in the particulate filter reaches a predetermined amount, fuel is supplied as an auxiliary agent into an exhaust passage, upstream of the particulate filter, to burn off the particulate matter trapped by the particulate filter, thereby regenerating the particulate filter and maintaining the particulate trap function. There are also known exhaust gas purification catalysts in which an auxiliary agent is supplied into the exhaust passage in like manner to maintain the exhaust gas purifying function.

For example, as a catalyst for converting NOx contained in exhaust gas into unharmful substances, there is known a NOx adsorption catalyst designed to adsorb NOx contained in exhaust gas when the air-fuel ratio of the exhaust gas is lean, and release and reduce the adsorbed NOx when the air-fuel ratio of the exhaust gas is rich. Since the capacity of the NOx adsorption catalyst to adsorb NOx has a limit, it is necessary to cause the adsorbed NOx to be released and reduced. Thus, from Japanese Unexamined Patent Publication No. 2000-205005 (hereinafter referred to as Patent Document 1), for example, there is known an exhaust gas purification device in which, in order to maintain the exhaust gas purifying function of the NOx adsorption catalyst by causing it to release and reduce the adsorbed NOx, a fuel addition valve is provided to the exhaust passage, upstream of the NOx adsorption catalyst, so that fuel required to cause the release and reduction of NOx is injected from the fuel addition valve into the exhaust passage and supplied to the NOx adsorption catalyst.

In the exhaust gas purification device shown in Patent Document 1, fuel in the amount required to cause the NOx adsorption catalyst to release and reduce the adsorbed NOx is injected into the exhaust passage, upstream of the NOx adsorption catalyst, by means of the fuel addition valve, so that exhaust gas with a rich air-fuel ratio enters the NOx adsorption catalyst and causes the NOx adsorption catalyst to release and reduce the adsorbed NOx. In this process, the amount of fuel supplied is regulated by varying the valve open time of the fuel addition valve, where the amount of fuel injected into the exhaust pipe increases as the valve open time becomes longer.

The amount of fuel required to cause the NOx adsorption catalyst to release and reduce the adsorbed NOx, or the valve open time of the fuel addition valve needs to be determined on the basis of NOx accumulation quantity, namely the amount of NOx adsorbed by the NOx adsorption catalyst, etc. It is however difficult to directly detect the NOx accumulation quantity. Thus, actually, the valve open time corresponding to the required fuel quantity is determined from a map set in advance to give the valve open time as a function of intake air quantity and engine revolution speed.

The amount of fuel injected from the fuel addition valve into the exhaust passage, however, varies depending on exhaust pressure in the exhaust passage and the temperature of fuel supplied. Thus, even when the valve open time corresponding to the required fuel quantity is accurately obtained on the basis of the intake air quantity and the engine revolution speed, the amount of fuel actually injected from the fuel addition valve into the exhaust passage can be different from the fuel quantity obtained from the map.

Specifically, if the supply pressure of fuel to be injected is fixed, pressure difference between the fuel supply pressure and the exhaust pressure is smaller when the exhaust pressure is higher, compared with when the exhaust pressure is lower. Thus, the amount of fuel actually supplied into the exhaust passage in the same valve open time becomes smaller as the exhaust pressure increases. Particularly when an exhaust throttle valve is provided in the exhaust passage for using it as an exhaust brake, or for such purpose as controlling the temperature of the exhaust gas purification catalyst or the particulate filter, the exhaust pressure varies to a great degree, depending on the opening and closing of the exhaust throttle valve, and therefore has a greater influence on the amount of fuel supplied.

Further, fuel has lower viscosity in the case where the temperature of fuel is higher, compared with that in the case where the temperature of fuel is lower. Thus, the amount of fuel actually supplied into the exhaust passage in the same valve open time becomes greater as the fuel temperature increases.

As stated above, the amount of fuel supplied from the fuel addition valve into the exhaust passage varies depending on the exhaust pressure and the fuel temperature. Thus, fuel in the amount required is not always supplied to the NOx adsorption catalyst, which leads to problems such that NOx is not sufficiently converted into unharmful substances by the NOx adsorption catalyst, that the adsorbed NOx is not sufficiently released so that the purifying capacity of the NOx adsorption catalyst lowers, and that fuel is added excessively so that excess fuel is emitted into the atmosphere.

DISCLOSURE OF THE INVENTION

The present invention has been made to solve the problems as mentioned above, and the primary object of the present invention is to provide an exhaust gas purification device for an internal combustion engine which can stably maintain an exhaust gas purifying function by accurately supplying an auxiliary agent required for maintaining the exhaust gas purifying function.

An exhaust gas purification device for an internal combustion engine according to the present invention comprises: an exhaust gas purification means provided in an exhaust passage of the internal combustion engine for purifying exhaust gas exhausted from the internal combustion engine; an auxiliary agent supply means for supplying an auxiliary agent for maintaining the exhaust gas purifying function of the exhaust gas purification means, into the exhaust passage, upstream of the exhaust gas purification means; a variation factor parameter detection means for detecting the value of a variation factor parameter which causes variations in the amount of the auxiliary agent supplied; and a control means for controlling the auxiliary agent supply means, thereby regulating the amount of the auxiliary agent supplied into the exhaust passage, wherein the control means includes a standard supply quantity determination section for determining standard supply quantity of the auxiliary agent required to maintain the exhaust gas purifying function of the exhaust gas purification means; a target supply quantity determination section for determining target supply quantity of the auxiliary agent, by correcting the standard supply quantity determined by the standard supply quantity determination section, on the basis of the value of the parameter detected by the variation factor parameter detection means; and a supply control section for controlling the auxiliary agent supply means to supply the auxiliary agent in the target supply quantity determined by the target supply quantity determination section.

In the exhaust gas purification device for the internal combustion engine according to the present invention, the standard supply quantity of the auxiliary agent required to maintain the exhaust gas purifying function of the exhaust gas purification means is corrected on the basis of the value of the variation factor parameter, and the auxiliary agent is supplied into the exhaust passage in the quantity corrected this way. Thus, the auxiliary agent can be supplied into the exhaust passage accurately in the amount required to maintain the exhaust gas purifying function of the exhaust gas purification means, obviating the influence of the variation factor on the amount of the auxiliary agent supplied.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the overall structure of an exhaust gas purification device for an internal combustion engine according to a first embodiment of the present invention;

FIG. 2 is a block diagram for light oil supply control by an ECU of FIG. 1;

FIG. 3 is a flow chart showing the light oil supply control by the ECU of FIG. 1;

FIG. 4 is a diagram showing a characteristic of an exhaust pressure Pex-correction factor Rp map used by the ECU of FIG. 1;

FIG. 5 is a diagram showing a characteristic of a light oil temperature Tf-correction factor Rt map used by the ECU of FIG. 1;

FIG. 6 is a diagram showing a characteristic of a target supply quantity Mt-duty cycle Dt map used by the ECU of FIG. 1;

FIG. 7 is a block diagram for light oil supply control in an exhaust gas purification device for an internal combustion engine according to a second embodiment of the present invention;

FIG. 8 is a diagram showing the schematic structure of an exhaust gas purification device for an internal combustion engine according to a third embodiment of the present invention;

FIG. 9 is a block diagram for urea water supply control in the exhaust gas purification device of FIG. 8;

FIG. 10 is a flow chart showing the urea water supply control in the exhaust gas purification device of FIG. 8;

FIG. 11 is a diagram showing a characteristic of an exhaust pressure Pex-correction factor Rp map used in the urea water supply control in the exhaust gas purification device of FIG. 8;

FIG. 12 is a diagram showing a characteristic of a urea water temperature Tu-correction factor Rp map used in the urea water supply control in the exhaust gas purification device of FIG. 8;

FIG. 13 is a diagram showing the schematic structure of an exhaust gas purification device for an internal combustion engine according to a fourth embodiment of the present invention; and

FIG. 14 is a block diagram showing light oil supply control in the exhaust gas purification device of FIG. 13.

BEST MODE OF CARRYING OUT THE INVENTION

With reference to the accompanying drawings, embodiments of the present invention will be described below.

FIG. 1 shows the system structure of a four-cylinder diesel engine (hereinafter referred to as an engine) to which an exhaust gas purification device according to a first embodiment of the present invention is applied. Referring to FIG. 1, the structure of the exhaust gas purification device according to the present invention will be described.

As shown in FIG. 1, the engine 1 is an inline four-cylinder diesel engine, in which fuel is directly supplied into each cylinder by means of a fuel injection valve (not shown) provided for each cylinder.

A turbocharger 4 is provided to an intake passage 2. Intake air sucked in through an air cleaner (not shown) flows from the intake passage 2 to a compressor 4 a of the turbocharger 4. After compressed by the compressor 4 a, the intake air is passed through an intercooler 10, and then introduced into an intake manifold 8.

An air flow sensor 10 for detecting the amount of intake air flow to the engine 1 is provided to the intake passage 2, upstream of the compressor 4 a. Further, an intake throttle valve 12 for regulating the amount of intake air taken into the engine 1 is provided in the intake passage 2, downstream of the intercooler 6.

Exhaust ports (not shown) through which exhaust gas is exhausted from the respective cylinders of the engine 1 are connected to an exhaust pipe (exhaust passage) 16 by means of an exhaust manifold 14. An EGR passage 20 is provided to connect the exhaust manifold 14 and the intake manifold 8, with an EGR valve 18 disposed between the exhaust manifold 14 and the intake manifold 8.

The exhaust pipe 16 is connected to an exhaust after-treatment device 22 through a turbine 4 b of the turbocharger 4. The turbine 4 b is coupled with the compressor 4 a, and receives exhaust gas flowing in the exhaust pipe 16, thereby driving the compressor 4 a.

The exhaust after-treatment device 22 has a casing, within which an NOx adsorption catalyst 24 as an exhaust gas purification means is disposed on the upstream side and a DPF (diesel particulate filter) 26 is disposed downstream of the NOx adsorption catalyst 24. The NOx adsorption catalyst 24 has a function of adsorbing NOx contained in exhaust gas when the exhaust air-fuel ratio is lean, and releasing and reducing the adsorbed NOx when the exhaust air-fuel ratio is rich. The NOx adsorption catalyst 24 having such function is in itself publicly known. The DPF 26 has a function of trapping particulate matter contained in exhaust gas. Also the DPF 26 is in itself publicly known. The exhaust gas purified by these NOx adsorption catalyst 24 and DPF 26 is emitted into the atmosphere.

Upstream of the exhaust after-treatment device 22, an exhaust throttle valve 28 functioning as an exhaust brake is provided, and upstream of the exhaust throttle valve 28, an exhaust pressure sensor (exhaust pressure detection means) 30 for detecting exhaust pressure in the exhaust pipe 16 is provided.

Further, in order to make the exhaust air-fuel ratio rich to cause the NOx adsorption catalyst 24 to release and reduce the adsorbed NOx, there is provided, upstream of the exhaust throttle valve 28, a light oil addition valve (auxiliary agent supply means) 32 for injecting light oil, which is the same fuel as that supplied to the engine 1, into the exhaust pipe 16, as an auxiliary agent. The light oil addition valve 32 is a solenoid valve designed to be opened to inject light oil by energizing a solenoid, and closed to stop supplying the light oil by stopping energizing the solenoid. Thus, when light oil supply pressure is fixed, light oil is supplied into the exhaust pipe 16 in the amount proportional to the time of energizing the light oil addition valve 32.

Light oil is supplied to the light oil addition valve 32 through a light oil supply passage 34. A light oil temperature sensor (auxiliary agent temperature detection means) 36 for detecting the temperature of the light oil supplied to the light oil addition valve 32 is provided to the light oil supply passage 34.

An ECU (control means) 38 is a control device for performing general control on the exhaust gas purification device according to the present invention, including control on the engine 1. The ECU comprises a CPU, memory devices, timer counters, etc. and determines the values of a variety of control variables, including fuel supply quantity for each cylinder, and controls a variety of devices on the basis of the control variable values determined.

To the input of the ECU 38, there are connected a variety of sensors including the air flow sensor 10, the exhaust pressure sensor 30 and the light oil temperature sensor 36 to collect information required for a variety of controls. To the output of the ECU 38, there are connected a variety of devices including the fuel injection valve (not shown) for each cylinder and the light oil addition valve 32, which devices are controlled on the basis of control variable values determined.

In the exhaust gas purification device for the internal combustion engine having the configuration described above, exhaust gas exhausted from the engine 1 during the operation of the engine 1 is introduced through the exhaust pipe 16 into the exhaust after-treatment device 22, where NOx contained in the exhaust gas is adsorbed by the NOx adsorption catalyst 24 and particulate matter contained in the exhaust gas is trapped by the DPF 26.

The removal of particulate matter is carried out as follows: Light oil is injected from the light oil addition valve 32 into the exhaust pipe 16 and oxidized on the NOx adsorption catalyst 24. The resulting high-temperature gas is forced to flow into the DPF 26, so that the particulate matter trapped by the DPF 26 is removed from the DPF 26 by oxidation.

The NOx which is not adsorbed but remains in the exhaust gas because the amount of NOx adsorbed by the NOx adsorption catalyst 24 has reached the limit enters the DPF 26 downstream of the NOx catalyst 24 and acts as an oxidizer to oxidize the particulate matter trapped by the DPF 26. Consequently, the particulate matter is removed from the DPF 26 by oxidation while the NOx flown into the DPF 26 turns into N₂, which is emitted into the atmosphere.

The conversion of NOx is carried out as follows: By carrying out lean operation, the NOx adsorption catalyst is caused to adsorb the NOx contained in the exhaust gas. After the amount of NOx adsorbed by the NOx adsorption catalyst 24 reaches a certain level, light oil is injected from the light oil addition valve 32 into the exhaust pipe 16 to make the exhaust air-fuel ratio rich. Supplied with exhaust gas with a rich air-fuel ratio obtained in this manner, the NOx adsorption catalyst 24 releases and reduces the adsorbed NOx, thereby restoring the adsorption capacity of the NOx adsorption catalyst 24. After the regeneration of the NOx adsorption catalyst 24 by release and reduction of the adsorbed NOx is completed, the injection of the light oil from the light oil addition valve 32 is terminated.

By appropriately repeating the regeneration of the NOx adsorption catalyst 24 and of the DPF 26, the exhaust gas purifying function of the NOx adsorption catalyst 24 and of the DPF 26 is maintained.

Next, referring to FIGS. 2 to 6, control of light oil supply from the light oil addition valve 32 will be described.

FIG. 2 shows the configuration of control blocks for carrying out the light oil supply control in the ECU 38, and FIG. 3 is a flow chart showing the light oil supply control performed by those control blocks.

As shown in FIG. 2, the ECU 38 includes a standard supply quantity determination section 40 for determining standard supply quantity Mb of the light oil required to cause the NOx adsorption catalyst 24 to release and reduce the NOx adsorbed by the NOx adsorption catalyst 24, thereby maintaining the NOx adsorption capacity of the NOx adsorption catalyst 24; a target supply quantity determination section 42 for determining target supply quantity Mt by correcting the standard supply quantity Mb determined by the standard supply quantity determination section 40, on the basis of exhaust pressure Pex detected by the exhaust pressure sensor 30 and light oil temperature Tf detected by the light oil temperature sensor 36; and a supply control section 44 for controlling the light oil addition valve 32 so that light oil is supplied into the exhaust passage in the target supply quantity Mt determined by the target supply quantity determination section 42.

More specifically, intake air flow rate Qa detected by the air flow sensor 10 and engine revolution speed Ne detected by the revolution speed sensor 46 are fed to the standard supply quantity determination section 40, and from a map stored in advance, standard supply quantity Mb of the light oil required to cause the NOx adsorption catalyst 24 to release and reduce the adsorbed NOx is determined on the basis of these intake air flow rate Qa and engine revolution speed Ne (Step S10 in FIG. 3).

The standard supply quantity Mb determined by the standard supply quantity determination section 40 is sent to the target supply quantity determination section 42. Exhaust pressure Pex detected by the exhaust pressure sensor 30 and light oil temperature Tf detected by the light oil temperature sensor 36 are fed to the target supply quantity determination section 42, and the target supply quantity determination section 42 corrects the standard supply quantity Mb on the basis of these exhaust pressure Pex and light oil temperature Tf.

The amount of light oil supplied from the light oil addition valve 32 is regulated by varying the valve open time thereof, where the amount of fuel injected into the exhaust pipe 16 increases as the valve open time becomes longer. Accordingly, if the light oil supply pressure is fixed, the amount of light oil actually supplied into the exhaust pipe 16 in the same valve open time becomes smaller as the exhaust pressure increases. Further, light oil has lower viscosity in the case where the temperature of the light oil is higher, compared with the case where the temperature of the light oil is lower. Thus, the amount of light oil actually supplied into the exhaust pipe 16 in the same valve open time becomes greater as the light oil temperature increases.

Thus, in connection with the exhaust pressure Pex, a correction factor Rp corresponding to the detected exhaust pressure Pex is determined from a map stored in advance (Step S12 in FIG. 3), where the map is prepared such that the correction factor Rp becomes smaller as the exhaust pressure Pex increases, as shown in FIG. 4. By dividing the standard supply quantity Mb by the correction factor Rp, the target supply quantity determination section 42 corrects the standard supply quantity Mb to obtain pressure-corrected supply quantity Mp (Step S14 in FIG. 3).

It is to be noted that the correction factor Rp is set to 1.0 in a standard state in which the exhaust pressure is equal to the exhaust pressure value based on which the map used to determine the standard supply quantity Mb is set.

By correcting the standard supply quantity Mb using the correction factor Rp in this manner, the pressure-corrected supply quantity Mp is greater than the standard supply quantity Mb when the exhaust pressure Pex is higher than the exhaust pressure value in the standard state. Thus, the shortage of supply quantity due to an increase in exhaust pressure Pex is compensated for. Conversely, the pressure-corrected supply quantity Mp is smaller than the standard supply quantity Mb when the exhaust pressure Pex is lower than the exhaust pressure value in the standard state. Thus, the excess of supply quantity due to a decrease in exhaust pressure Pex is obviated.

Next, in connection with the light oil temperature Tf, a correction factor Rt corresponding to the detected light oil temperature Tf is determined from a map stored in advance (Step S16 in FIG. 3), where the map is prepared such that the correction factor Rt becomes greater as the light oil temperature Tf increases, as shown in FIG. 5. By dividing the pressure-corrected supply quantity Mp by the correction factor Rt, the target supply quantity determination section 42 corrects the pressure-corrected supply quantity Mp to obtain target supply quantity Mt (Step S18 in FIG. 3).

It is to be noted that the correction factor Rt is set to 1.0 in a standard state in which the light oil temperature is equal to the light oil temperature value based on which the map used to determine the standard supply quantity Mb is set.

Here, the correction using the correction factor Rt is made to the pressure-corrected supply quantity Mp. However, since the pressure-corrected supply quantity Mp results from correcting the standard supply quantity Mb on the basis of the exhaust pressure Pex as mentioned above, the correction using the correction factor Rt is essentially made to the standard supply quantity Mb. Accordingly, by correcting the pressure-corrected supply quantity Mp, or essentially, the standard supply quantity Mb using the correction factor Rt in this manner, the target supply quantity Mt becomes smaller as the light oil temperature Tf increases, and thereby the excess of supply quantity due to a rise in light oil temperature Tf is obviated. Conversely, the target supply quantity Mt becomes greater as the light oil temperature Tf decreases, and thereby the shortage of supply quantity due to a drop in light oil temperature Tf is compensated for.

In the flow chart of FIG. 3, first at Steps S12 and S14, the pressure-corrected supply quantity Mp is obtained by correcting the standard supply quantity Mb on the basis of the exhaust pressure Pex, and then at Steps S16 and S18, the target supply quantity Mt is determined by correcting the pressure-corrected supply quantity Mp on the basis of the light oil temperature Tf. It is to be noted, however, that the order of the steps is not limited to this.

For example, Steps 12 and S14 can be interchanged with Steps S16 and S18. Specifically, it can be arranged such that first, temperature-corrected supply quantity is obtained by correcting the standard supply quantity Mb using the correction factor Rt corresponding to the light oil temperature Tf, and then the target supply quantity Mt is obtained by correcting the temperature-corrected supply quantity using the correction factor Rp corresponding to the exhaust pressure Pex.

Alternatively, it can be arranged such that first, the correction factor Rp corresponding to the exhaust pressure Pex and the correction factor Rt corresponding to the light oil temperature Tf are determined from the respective maps, and then, the target supply quantity Mt is obtained by dividing the standard supply quantity Mb by the correction factors Rp and Rt, successively.

Further, although in the above-described case, correction is made by dividing the standard supply quantity Mb, the pressure-corrected supply quantity Mp or the temperature-corrected supply quantity by the correction factor Rp or the correction factor Rt, it can be arranged such that the reciprocal of each correction factor is obtained from a map stored in advance so that correction is made by multiplying the supply quantity by the reciprocal.

After the target supply quantity Mt of the light oil required to cause the NOx adsorption catalyst 24 to release and reduce the adsorbed NOx is determined in this manner, the supply control section 44 determines, from a map stored in advance, valve open time of the light oil addition valve 32 required for the light oil addition valve 32 to inject light oil in the target supply quantity Mt (Step S20 in FIG. 3). Since the control on the light oil addition valve 32 is performed in control cycles of a predetermined period, the map is prepared to give the valve open time of the light oil addition valve 32 corresponding to the target supply quantity Mt, in the form of duty cycle Dt relative to the maximum valve open time in one control cycle, as shown in FIG. 6.

After determining the duty cycle Dt corresponding to the target supply quantity Mt from the map, the supply control section 44 drives the light oil addition valve 32 to open according to the duty cycle Dt (Step S22 in FIG. 3), so that light oil in a quantity equivalent to the target supply quantity Mt is injected from the light oil addition valve 32 into the exhaust pipe 16. This makes the exhaust air-fuel ratio rich, so that the NOx adsorbed by the NOx adsorption catalyst 24 is properly released and reduced.

It is to be noted that the exhaust pressure sensor 30 is disposed upstream of the exhaust throttle 28. Thus, even when the pressure in the exhaust pipe 16 varies due to the opening and closing of the exhaust throttle 28, light oil is always supplied properly in the amount required to cause the NOx adsorption catalyst 24 to release and reduce the adsorbed NOx, in spite of variations in pressure in the exhaust pipe 16, since, as mentioned above, the standard supply quantity Mb is corrected on the basis of the exhaust pressure detected by this exhaust pressure sensor 30.

As described above, in the exhaust gas purification device according to the first embodiment of the present invention, the amount of supply of light oil required to cause the NOx adsorption catalyst 24 to release and reduce the adsorbed NOx, thereby maintaining the NOx adsorption capacity of the NOx adsorption catalyst 24 is controlled properly, without being affected by variations in exhaust pressure and in light oil temperature. Thus, the exhaust gas purifying function can be stably maintained, and the emission of excess light oil into the atmosphere can be prevented.

It is to be noted that although in the exhaust gas purification device according to the above-described first embodiment, the target supply quantity Mt is determined by correcting the standard supply quantity Mb of the light oil required to maintain the NOx adsorption capacity of the NOx adsorption catalyst 24, on the basis of both the exhaust pressure Pex and the light oil temperature Tf, the correction may be made on the basis of either of them. The control accuracy in this case is lower, compared with when the correction is made on the basis of both the exhaust pressure Pex and the light oil temperature Tf, but higher, compared with the conventional exhaust gas purification device which takes account of neither of the exhaust pressure and the light oil temperature.

Further, although in the above-described first embodiment, the standard supply quantity Mb of the light oil required to cause the NOx adsorption catalyst 24 to release and reduce the adsorbed NOx is determined from a map stored in advance, on the basis of the intake air flow rate Qa and the engine revolution speed Ne, the way to determine the standard supply quantity Mb is not limited to this. For example, the standard supply quantity Mb may be determined on the basis of a decrease in NOx adsorption capacity which can be detected by a NOx sensor provided downstream of the NOx adsorption catalyst 24. There are a variety of known techniques that can be used.

Further, although the above-described first embodiment of the present invention is an exhaust gas purification device applied to the diesel engine, it is not limited to the diesel engine but applicable to any types of engines using a NOx adsorption catalyst. In the case of a gasoline engine, gasoline is used as an auxiliary agent, in place of light oil.

The NOx adsorption catalyst 24 used in the above-described exhaust gas purification device according to the first embodiment adsorbs SOx (sulfur oxides) produced by combustion of sulfur in fuel, which results in a deterioration in NOx adsorption function. Thus, it is necessary to restore the deteriorated NOx adsorption function by causing the NOx adsorption catalyst 24 to release the SOx adsorbed by the NOx adsorption catalyst 24. The SOx adsorbed by the NOx adsorption catalyst 24 can be released from the NOx adsorption catalyst 24 by raising the temperature of the NOx adsorption catalyst 24, and the temperature of the NOx adsorption catalyst 24 can be raised by supplying light oil to the NOx adsorption catalyst 24 by means of the light oil addition valve 32 used in the first embodiment and burning it.

Next, as a second embodiment of the present invention, an exhaust gas purification device designed to release the SOx adsorbed by the NOx adsorption catalyst 24 in this manner will be described.

Since the overall system structure is as shown in FIG. 1, namely the same as the first embodiment, the same reference signs will be used for the same elements as those of the first embodiment, for which a detailed description will be omitted, and essential elements different from the first embodiment will be mainly described bellow.

FIG. 7 shows the configuration of control blocks arranged in an ECU 38 (control means), which carries out oil supply control for releasing SOx.

As shown in FIG. 7, the ECU 38 includes a standard supply quantity determination section 50 for determining standard supply quantity Mb of the fuel required to cause the NOx adsorption catalyst 24 to release sulfur compound adsorbed by the NOx adsorption catalyst 24, thereby restoring the deteriorating NOx adsorption capacity; a target supply quantity determination section 52 for determining target supply quantity Mt by correcting the standard supply quantity Mb determined by the standard supply quantity determination section 50, on the basis of exhaust pressure Pex detected by the exhaust pressure sensor 30 and light oil temperature Tf detected by the light oil temperature sensor 36; and a supply control section 54 for controlling the light oil addition valve 32 so that light oil is supplied into the exhaust passage in the target supply quantity Mt determined by the target supply quantity determination section 52.

An exhaust temperature sensor 48 for detecting the temperature of exhaust gas entering the NOx adsorption catalyst 24 is connected to the standard supply quantity determination section 50. The standard supply quantity determination section 50 estimates SOx accumulation quantity, i.e., the amount of SOx adsorbed by the NOx adsorption catalyst 24, from cumulative fuel supply quantity for each cylinder which is calculated within the ECU 38, and determines standard supply quantity Mb of the light oil required to raise the temperature of the NOx adsorption catalyst 24 to an optimal temperature (600° C., for example) for releasing SOx, from a map stored in advance, on the basis of this estimated SOx accumulation quantity and the exhaust temperature Tex detected by the exhaust temperature sensor 48.

Like the above-described first embodiment, correction of the standard supply quantity Mb determined by the standard supply quantity determination section 50 and control of light oil injection from the light oil addition valve 32 are carried out according to a flow chart including the same steps as Steps S12 to S22 of the flow chart of FIG. 3.

Specifically, the target supply quantity determination section 52 receives the standard supply quantity Mb from the standard supply quantity determination section 50 and corrects the standard supply quantity Mb on the basis of the exhaust pressure Pex detected by the exhaust pressure sensor 30 and the light oil temperature Tf detected by the light oil temperature sensor 36.

The correction of the standard supply quantity Mb is performed in the same manner as in the first embodiment. In connection with the exhaust pressure Pex, a correction factor Rp corresponding to the detected exhaust pressure Pex is determined from a map stored in advance (Step S12 in FIG. 3), where the map is prepared such that the correction factor Rp becomes smaller as the exhaust pressure Pex increases, as shown in FIG. 4. By dividing the standard supply quantity Mb by the correction factor Rp, the target supply quantity determination section 52 corrects the standard supply quantity Mb to obtain pressure-corrected supply quantity Mp (Step S14 in FIG. 3).

By correcting the standard supply quantity Mb using the correction factor Rp in this manner, the pressure-corrected supply quantity Mp is greater than the standard supply quantity Mb when the exhaust pressure Pex is higher than the exhaust pressure value of the standard state. Thus, the shortage of supply quantity due to an increase in exhaust pressure Pex is compensated for. Conversely, the pressure-corrected supply quantity Mp is smaller than the standard supply quantity Mb when the exhaust pressure Pex is lower than the exhaust pressure value of the standard state. Thus, the excess of supply quantity due to a decrease in exhaust pressure Pex is obviated.

In connection with the light oil temperature Tf, a correction factor Rt corresponding to the detected light oil temperature Tf is determined from a map stored in advance (Step S16 in FIG. 3), where the map is prepared such that the correction factor Rt becomes greater as the light oil temperature Tf increases, as shown in FIG. 5. By dividing the pressure-corrected supply quantity Mp by the correction factor Rt, the target supply quantity determination section 52 corrects the pressure-corrected supply quantity Mp to obtain target supply quantity Mt (Step S18 in FIG. 3).

Here, the correction using the correction factor Rt is made to the pressure-corrected supply quantity Mp. However, as mentioned in respect of the first embodiment, since the pressure-corrected supply quantity Mp results from correcting the standard supply quantity Mb on the basis of the exhaust pressure Pex, the correction using the correction factor Rt is essentially made to the standard supply quantity Mb. Accordingly, by correcting the pressure-corrected supply quantity Mp, or essentially, the standard supply quantity Mb using the correction factor Rt in this manner, the target supply quantity Mp becomes smaller as the light oil temperature Tf increases. Thus, the excess of supply quantity due to a rise in light oil temperature Tf is obviated. Conversely, the target supply quantity Mt becomes greater as the light oil temperature Tf decreases. Thus, the shortage of supply quantity due to a drop in light oil temperature is compensated for.

It is to be noted that in the flow chart of FIG. 3, the order of Steps S12, S14 and Steps S16, S18 is not limited to this, as in the first embodiment.

Further, although in the above-described case, correction is made by dividing the standard supply quantity Mb, the pressure-corrected supply quantity Mp or the temperature-corrected supply quantity by the correction factor Rp or the correction factor Rt, it can be arranged such that the reciprocal of each correction factor is obtained from a map stored so that correction is made by multiplying the supply quantity by the reciprocal.

After the target supply quantity Mt of the light oil required to cause the NOx adsorption catalyst 24 to release the adsorbed SOx is determined in this manner, the supply control section 54 determines, from a map stored in advance, valve open time of the light oil addition valve 32 required for the light oil addition valve 32 to inject light oil in the target supply quantity Mt, in the form of duty cycle Dt (Step S20 in FIG. 3), as in the first embodiment.

After determining the duty cycle Dt corresponding to the target supply quantity Mt from the map, the supply control section 54 drives the light oil addition valve 32 to open according to the duty cycle Dt thus determined (Step S22 in FIG. 3), so that light oil in a quantity equivalent to the target supply quantity Mt is injected from the light oil addition valve 32 into the exhaust pipe 16. Due to the exhaust heat, the light oil in the exhaust gas decomposes into HC, which reaches the NOx adsorption catalyst and burns. This causes a rise in temperature of the NOx adsorption catalyst 24, so that SOx adsorbed by the NOx adsorption catalyst 24 is released properly and the NOx adsorption capacity of the NOx adsorption catalyst 24 is restored.

As described above, in the exhaust gas purification device according to the second embodiment of the present invention, the amount of supply of light oil required to cause the NOx adsorption catalyst 24 to release the adsorbed SOx, thereby maintaining the NOx adsorption capacity of the NOx adsorption catalyst 24 is controlled properly, without being affected by variations in exhaust pressure and in light oil temperature. Thus, the exhaust gas purifying function of the NOx adsorption catalyst 24 can be stably maintained, and the emission of excess light oil into the atmosphere can be prevented.

It is to be noted that although in the exhaust gas purification device according to the second embodiment, the target supply quantity Mt is determined by correcting the standard supply quantity Mb of the light oil required to maintain the NOx adsorption capacity of the NOx adsorption catalyst 24, on the basis of both the exhaust pressure Pex and the light oil temperature Tf, the correction may be made on the basis of either of them. The control accuracy in this case is lower, compared with when the correction is made on the basis of both the exhaust pressure Pex and the light oil temperature Tf, but higher, compared with the conventional exhaust gas purification device which takes account of neither of the exhaust pressure and the light oil temperature.

Further, although in the above-described case, the standard supply quantity Mb of the light oil required to cause the NOx adsorption catalyst 24 to release SOx is determined from a map stored in advance, on the basis of the cumulative fuel supply quantity for each cylinder and the exhaust temperature Tex, the way to determine the standard supply quantity Mb is not limited to this. There are a variety of known techniques that can be used.

It is also possible to arrange the first embodiment of the exhaust gas purification device to also carry out the SOx release control in the second embodiment, in a manner such that the supply of the light oil for release and reduction of NOx and the supply of the light oil for release of SOx are both carried out by means of the same light oil addition valve 32.

Further, although the above-described second embodiment of the present invention is an exhaust gas purification device applied to the diesel engine, it is not limited to the diesel engine but applicable to any types of engines using an NOx adsorption catalyst. In the case of a gasoline engine, gasoline is used as an auxiliary agent, in place of light oil.

Next, referring to FIGS. 8 to 11, a third embodiment of the present invention will be described.

FIG. 8 shows the structure of an exhaust gas purification device according to the third embodiment of the present invention. The structure of an engine, which forms a base for the exhaust gas purification device, and an intake system of the engine is the same as that for the first embodiment. In FIG. 8, the same reference signs are used for the same elements as those of the first embodiment.

In an exhaust pipe 16 connected to an exhaust manifold (not shown) of an engine, a turbine (not shown) of a turbocharger is incorporated, and downstream of the turbine, a selective reduction type NOx catalyst (hereinafter referred to as an SCR catalyst) 56 as an exhaust gas purification means is connected to the exhaust pipe 16. The SCR catalyst 56 promotes denitration reaction between ammonia and NOx contained in exhaust gas to selectively reduce NOx, namely convert NOx to unharmful substances.

An exhaust throttle valve 28 functioning as an exhaust brake is provided upstream of the SCR catalyst 56, and an exhaust pressure sensor (exhaust pressure detection means) 30 for detecting exhaust pressure in the exhaust pipe 16 is provide upstream of the exhaust throttle valve 28.

Further, in order to supply ammonia required for conversion of NOx to the SCR catalyst 56, a urea water addition valve (auxiliary agent supply means) 58 for injecting urea water as an auxiliary agent into the exhaust pipe 16 is provided upstream of the exhaust throttle valve 28. The urea water addition valve 58 is a solenoid valve designed to be opened to inject urea water by energizing a solenoid, and closed to stop injecting the urea water by stopping energizing the solenoid. Thus, when urea water supply pressure is fixed, urea water is supplied into the exhaust pipe 16 in the amount corresponding to the time of energizing the urea water addition valve 58.

Due to the heat of the exhaust gas, the urea water injected from the urea water addition valve 58 into the exhaust pipe 68 hydrolyzes into ammonia, which is supplied to the SCR catalyst 56 and used for conversion of NOx.

Urea water is supplied to the urea water addition valve 58 from a urea water storage tank (not shown), through a urea water supply passage 60. A urea water temperature sensor (auxiliary agent temperature detection means) 62 for detecting the temperature of the urea water supplied to the urea water addition valve 58 is provided to the urea water supply passage 60.

An upstream exhaust temperature sensor 64 for detecting the temperature of exhaust gas entering the SCR catalyst 56 is provided to the exhaust pipe 16, upstream of the SCR catalyst 56. Further, a downstream exhaust temperature sensor 66 for detecting the temperature of exhaust gas exiting the SCR catalyst 56 is provided to the exhaust pipe 16, downstream of the SCR catalyst 56.

As in the above-described first embodiment, to the input of an ECU (control means) 38, which is a control device for performing general control on the exhaust gas purification device according to the present invention, including control on the engine, there are connected a variety of sensors including the exhaust pressure sensor 30, the urea water temperature sensor 62, the upstream exhaust temperature sensor 64 and the downstream exhaust temperature sensor 66 to collect information required for a variety of controls. To the output of the ECU 38, there are connected a variety of devices including the fuel injection valve (not shown) for each cylinder and the urea water addition valve 58, which devices are controlled on the basis of control variable values determined in the ECU 38.

In the exhaust gas purification device having the configuration described above, exhaust gas exhausted from the engine is introduced through the exhaust pipe 16 into the SCR catalyst 56, while the urea water injected from the urea water addition valve 58 into the exhaust pipe 68 hydrolyzes into ammonia due to the heat of the exhaust gas, and the ammonia is supplied to the SCR catalyst 56. On the SCR catalyst 56, denitration reaction between ammonia and NOx in exhaust gas is promoted, so that NOx is converted to unharmful substances.

By supplying urea water into the exhaust pipe 16 as an auxiliary agent in this manner, the exhaust gas purifying function of the SCR catalyst 56 is maintained.

Next, referring to FIGS. 9 to 12, control of urea water supply from the urea water addition valve 58 will be described.

FIG. 9 shows the configuration of control blocks arranged in the ECU 38 for carrying out the urea water supply control, and FIG. 10 is a flow chart showing the urea water supply control.

As shown in FIG. 9, the ECU 38 includes a standard supply quantity determination section 68 for determining standard supply quantity Mb of the urea water required for the SCR catalyst 56 to selectively reduce NOx contained in exhaust gas; a target supply quantity determination section 70 for determining target supply quantity Mt by correcting the standard supply quantity Mb determined by the standard supply quantity determination section 68, on the basis of exhaust pressure Pex detected by the exhaust pressure sensor 30 and urea water temperature Tu detected by the urea water temperature sensor 62; and a supply control section 72 for controlling the urea water addition valve 58 so that urea water is supplied into the exhaust pipe 16 in the target supply quantity Mt determined by the target supply quantity determination section 70.

More specifically, exhaust temperature Texu upstream of the SCR catalyst 56 detected by the upstream exhaust temperature sensor 64, exhaust temperature Texd downstream of the SCR catalyst 56 detected by the downstream exhaust temperature sensor 66, and engine revolution speed Ne detected by the revolution speed sensor 46 are fed to the standard supply quantity determination section 68. From a map stored in advance, standard supply quantity Mb of the urea water required for the SCR catalyst 56 to selectively reduce NOx contained in exhaust gas is determined on the basis of fuel supply quantity for each cylinder which is calculated within the ECU 38, estimated NOx emission quantity and NOx conversion ratio which are determined from maps stored in advance, exhaust temperature Texu, exhaust temperature Texd and engine revolution speed Ne which are fed from the above-mentioned sensors, etc. (Step S110 in FIG. 10).

The above-mentioned technique for the standard supply quantity determination section 68 to determine the standard supply quantity Mb is in itself publicly known, and the manner of determining the standard supply quantity Mb of the urea water required for the SCR catalyst 56 to selectively reduce NOx contained in exhaust gas is not limited to this.

The standard supply quantity Mb determined by the standard supply quantity determination section 68 is sent to the target supply quantity determination section 70. To the target supply quantity determination section 70, exhaust pressure Pex detected by the exhaust pressure sensor 30 and urea water temperature Tu detected by the urea water temperature sensor 62 are fed, and the target supply quantity determination section 70 corrects the standard supply quantity Mb on the basis of these exhaust pressure Pex and urea water temperature Tu.

As in the first embodiment, the amount of urea water injected from the urea water addition valve 58 is regulated by varying the valve open time of the urea water addition valve 58, where the amount of urea water injected into the exhaust pipe 16 increases as the valve open time becomes longer. Accordingly, when the urea water supply pressure is fixed, the amount of urea water actually supplied into the exhaust pipe 16 in the same valve open time becomes smaller as the exhaust pressure increases. Further, urea water has lower viscosity in the case where the temperature of urea water is higher, compared with the case where the temperature of urea water is lower. Accordingly, the amount of urea water actually supplied into the exhaust pipe 16 in the same valve open time becomes greater as the urea water temperature increases.

Thus, in connection with the exhaust pressure Pex, a correction factor Rp corresponding to the detected exhaust pressure Pex is determined from a map stored in advance (Step S112 in FIG. 10), where the map is prepared such that the correction factor Rp becomes smaller as the exhaust pressure Pex increases, as shown in FIG. 11. By dividing the standard supply quantity Mb by the correction factor Rp, the target supply quantity determination section 70 corrects the standard supply quantity Mb to obtain pressure-corrected supply quantity Mp (Step S114 in FIG. 10).

It is to be noted that the correction factor Rp is set to 1.0 in a standard state in which the exhaust pressure is equal to the exhaust pressure value based on which the map used to determine the standard supply quantity Mb is set.

By correcting the standard supply quantity Mb using the correction factor Rp in this manner, the pressure-corrected supply quantity Mp is greater than the standard supply quantity Mb when the exhaust pressure Pex is higher than the exhaust pressure value of the standard state. Thus, the shortage of supply quantity due to an increase in exhaust pressure Pex is compensated for. Conversely, the pressure-corrected supply quantity Mp is smaller than the standard supply quantity Mb when the exhaust pressure Pex is lower than the exhaust pressure value of the standard state. Thus, the excess of supply quantity due to a decrease in exhaust pressure Pex is obviated.

Next, in connection with the urea water temperature Tu, a correction factor Rt corresponding to the detected urea water temperature Tu is determined from a map stored in advance (Step S116 in FIG. 10), where the map is prepared such that the correction factor Rt becomes greater as the urea water temperature Tu increases, as shown in FIG. 12. By dividing the pressure-corrected supply quantity Mp by the correction factor Rt, the target supply quantity determination section 70 corrects the pressure-corrected supply quantity Mp to obtain target supply quantity Mt (Step S118 in FIG. 10).

It is to be noted that the correction factor Rt is set to 1.0 in a standard state in which the urea water temperature is equal to the urea water temperature value based on which the map used to determine the standard supply quantity Mb is set.

Here, the correction using the correction factor Rt is made to the pressure-corrected supply quantity Mp. However, since the pressure-corrected supply quantity Mp results from correcting the standard supply quantity Mb on the basis of the exhaust pressure Pex as mentioned above, the correction using the correction factor Rt is essentially made to the standard supply quantity Mb. Accordingly, by correcting the pressure-corrected supply quantity Mp, or essentially, the standard supply quantity Mb using the correction factor Rt in this manner, the target supply quantity Mt becomes smaller as the urea water temperature Tu increases. Thus, the excess of supply quantity due to a rise in urea water temperature Tf is obviated. Conversely, the target supply quantity Mp becomes greater as the urea water temperature Tu decreases. Thus, the shortage of supply quantity due to a drop in urea water temperature Tu is compensated for.

In the flow chart of FIG. 10, as stated above, first at Steps S112 and S114, the pressure-corrected supply quantity Mp is obtained by correcting the standard supply quantity Mb on the basis of the exhaust pressure Pex, and then at Steps S116 and S118, the target supply quantity Mt is determined by correcting the pressure-corrected supply quantity Mp on the basis of the urea water temperature Tu. It is to be noted, however, that the order of the steps is not limited to this, like the first embodiment.

For example, Steps S112 and S114 can be interchanged with Steps S116 and S118. Specifically, it can be arranged such that first, temperature-corrected supply quantity is obtained by correcting the standard supply quantity Mb using the correction factor Rt corresponding to the urea water temperature Tu, and then the target supply quantity Mt is obtained by correcting the temperature-corrected supply quantity using the correction factor Rp corresponding to the exhaust pressure Pex.

Alternatively, it can be arranged such that first, the correction factor Rp corresponding to the exhaust pressure Pex and the correction factor Rt corresponding to the urea water temperature Tu are determined from the respective maps, and then, the target supply quantity Mt is obtained by dividing the standard supply quantity Mb by the correction factors Rp and Rt, successively.

Further, although in the above-described case, correction is made by dividing the standard supply quantity Mb, the pressure-corrected supply quantity Mp or the temperature-corrected supply quantity by the correction factor Rp or the correction factor Rt, it can be arranged such that the reciprocal of each correction factor is obtained from a map stored in advance so that correction is made by multiplying the supply quantity by the reciprocal.

After the target supply quantity Mt of the urea water required for the SCR catalyst 56 to selectively reduce NOx is determined in this manner, the supply control section 72 determines, from a map stored in advance, valve open time of the urea water addition valve 58 required for the urea water addition valve 58 to inject urea water in the target supply quantity Mt (Step S120 in FIG. 10). As in the first embodiment, since the control on the urea water addition valve 58 is performed in control cycles of a predetermined period, the map is prepared to give the valve open time of the urea water addition valve 58 corresponding to the target supply quantity Mt, in the form of duty cycle Dt relative to the maximum valve open time in one control cycle. Although graphical representation is omitted, the relation between the target supply quantity Mt and the duty cycle Dt is a proportional one similar to that shown in FIG. 6 in respect of the first embodiment.

After determining the duty cycle Dt corresponding to the target supply quantity Mt from the map, the supply control section 72 drives the urea water addition valve 58 to open according to the determined duty cycle Dt (Step S122 in FIG. 10), so that urea water in a quantity equivalent to the target supply quantity Mt is injected from the urea water addition valve 58 into the exhaust pipe 16. Due to the heat of the exhaust gas, the urea water injected into the exhaust pipe 16 in this manner hydrolyzes into ammonia, which acts as a reducing agent to selectively reduce NOx contained in exhaust gas on the SCR catalyst 56.

It is to be noted that the exhaust pressure sensor 30 is disposed upstream of the exhaust throttle valve 28. Thus, even when the pressure in the exhaust pipe 16 varies due to the opening and closing of the exhaust throttle valve 28, urea water is always supplied properly in the amount required for the SCR catalyst 56 to selectively reduce NOx, in spite of variations in pressure in the exhaust pipe 16, since, as mentioned above, the standard supply quantity Mb is corrected on the basis of the exhaust pressure Pex detected by this exhaust pressure sensor 30.

As described above, in the exhaust gas purification device according to the third embodiment of the present invention, the amount of supply of urea water required for the SCR catalyst 56 to selectively reduce NOx contained in exhaust gas, thereby maintaining its NOx conversion capacity is controlled properly, without being affected by variations in exhaust pressure and in urea water temperature. Thus, the exhaust gas purifying function can be stably maintained, and the emission of excess urea water or ammonia into the atmosphere can be prevented.

It is to be noted that although in the above-described exhaust gas purification device according to the third embodiment, the target supply quantity Mt is determined by correcting the standard supply quantity Mb of the urea water required to maintain the NOx conversion capacity of the SCR catalyst 56, on the basis of both the exhaust pressure Pex and the urea water temperature Tu, the correction can be made on the basis of either of them. The control accuracy in this case is lower, compared with when the correction is made on the basis of both the exhaust pressure Pex and the urea water temperature Tu, but higher, compared with the conventional exhaust gas purification device which takes account of neither of the exhaust pressure and the urea water temperature.

Further, although the above-described third embodiment of the present invention is an exhaust gas purification device applied to the diesel engine, it is not limited to the diesel engine but applicable to any types of engines using an SCR catalyst to selectively reduce NOx contained in exhaust gas by supplying urea water.

Next, referring to FIGS. 13 to 14, an exhaust gas purification device according to a fourth embodiment of the present invention will be described.

FIG. 13 shows the structure of an exhaust gas purification device according to the fourth embodiment of the present invention. The structure of an engine, which forms a base for the exhaust gas purification device, and an intake system is the same as that for the first embodiment. In FIG. 13, the same reference signs are used for the same elements as those of the first embodiment.

In an exhaust pipe 16 connected to an exhaust manifold (not shown) of an engine, a turbine (not shown) of a turbocharger is incorporated, and downstream of the turbine, an exhaust after-treatment device 74 is connected to the exhaust pipe 16.

The exhaust after-treatment device 74 has a casing, within which an oxidation catalyst 76 is disposed on the upstream side and a DPF (diesel particulate filter) 78 as an exhaust gas purification means is disposed downstream of the oxidation catalyst 76. The DPF 78 has a porous honeycomb structure made of ceramic, and has a function of trapping particulate matter contained in exhaust gas when the exhaust gas flows through it.

The particulate matter trapped by the DPF 78 accumulates on the DPF 78, which causes a gradual decrease in the trap capacity of the DPF 78 and an increase in resistance exerted on the flowing exhaust gas. Thus, it is necessary to burn off the particulate matter when the accumulation of the particulate matter reaches a certain level, thereby maintaining the trap capacity of the DPF 78. In order to raise the exhaust temperature to a level at which the particulate matter can be burned off, the oxidation catalyst 76 is used. Specifically, light oil is supplied as an auxiliary agent to the oxidation catalyst 76 in a manner described later and burned, and this combustion of light oil raises the exhaust temperature, so that the particulate matter accumulated in the DPF 78 is removed by burning.

To the after-treatment device 74, an inlet temperature sensor 80 for detecting exhaust temperature Tin at the inlet side of the DPF 78 and an inlet pressure sensor 82 for detecting exhaust pressure Pin at the inlet side of the DPF 78 are provided between the oxidation catalyst 76 and the DPF 78, and an outlet pressure sensor 84 for detecting exhaust pressure Pout at the outlet side of the DPF 78 is provided downstream of the DPF 78.

Upstream of the exhaust after-treatment device 74, an exhaust throttle valve 28 functioning as an exhaust brake is provided, and upstream of the exhaust throttle valve 28, an exhaust pressure sensor (exhaust pressure detection means) 30 for detecting exhaust pressure in the exhaust pipe 16 is provided.

Further, in order to supply the light oil required to burn off the particulate matter accumulated in the DPF 78, to the oxidation catalyst 76, a light oil addition valve (auxiliary agent supply means) 32 for injecting light oil as an auxiliary agent into the exhaust pipe 16 is provided upstream of the exhaust throttle valve 28. This light oil addition valve 32 is of the same type as that used in the first embodiment, and designed to be opened to inject light oil by energizing a solenoid, and closed to stop injecting the light oil by stopping energizing the solenoid. Thus, when light oil supply pressure is fixed, light oil is supplied into the exhaust pipe 16 in the amount according to the time of energizing the light oil addition valve 32.

The same light oil as that supplied to each cylinder of the engine is supplied to the light oil addition valve 32 through a light oil supply passage 34. A light oil temperature sensor (auxiliary agent temperature detection means) 36 for detecting the temperature of the light oil supplied to the light oil addition valve 32 is provided to the light oil supply passage 34.

As in the first embodiment, to the input of an ECU (control means) 38, which is a control device for performing general control on the exhaust gas purification device according to the present invention, including control on the engine, there are connected a variety of sensors including the exhaust pressure sensor 30, the inlet temperature sensor 80, the inlet pressure sensor 82 and the outlet temperature sensor 84 to collect information required for a variety of controls. To the output of the ECU 38, there are connected a variety of devices including the fuel injection valve (not shown) for each cylinder and the light oil addition valve 32, which devices are controlled on the basis of control variable values determined in the ECU 38.

In the exhaust gas purification device having the configuration described above, exhaust gas exhausted from the engine is introduced through the exhaust pipe 16 into the exhaust after-treatment device 74, and when the exhaust gas flows through the DPF 78, particulate matter contained in the exhaust gas is trapped and accumulated in the DPF 78. When, from a difference between a value detected by the inlet pressure sensor 82 and a value detected by the outlet pressure sensor 84, etc., it is determined that the amount of particulate matter accumulated in the DPF 78 has reached a predetermined level, light oil is injected from the light oil addition valve 32 into the exhaust pipe 16, as an auxiliary agent. Due to the heat of the exhaust gas, the light oil injected decomposes into HC, which is supplied to the oxidation catalyst 76, and oxidation reaction of the HC is promoted by the oxidation catalyst so that the HC burns. This combustion of HC raises the exhaust gas entering the DPF 78 to a temperature suited for burning-off of the particulate matter accumulated in the DPF 78 (500° C., for example). Consequently, the particulate matter accumulated in the DPF 78 is removed, so that the particulate trap function that has lowered is restored and the exhaust gas purifying function of the DPF 78 is maintained.

Next, referring to FIG. 14, control of light oil supply from the light oil addition valve 32 will be described. FIG. 14 shows the configuration of control blocks arranged in the ECU 38 for carrying out the light oil supply control.

As shown in FIG. 14, the ECU 38 includes a standard supply quantity determination section 86 for determining standard supply quantity of the light oil required to burn off particulate matter trapped by the DPF 78, thereby regenerating the DPF 78; a target supply quantity determination section 88 for determining target supply quantity Mt by correcting the standard supply quantity Mb determined by the standard supply quantity determination section 86, on the basis of exhaust pressure Pex detected by the exhaust pressure sensor 30 and light oil temperature Tf detected by the light oil temperature sensor 62; and a supply control section 90 for controlling the light oil addition valve 32 so that light oil is supplied into the exhaust pipe 16 in the target supply quantity Mt determined by the target supply quantity determination section 88.

More specifically, exhaust pressure Pin at the inlet side of the DPF 78, which is detected by the inlet pressure sensor 82, exhaust pressure Pout at the outlet side of the DPF 78, which is detected by the outlet pressure sensor 84, and exhaust temperature Tin at the inlet side of the DPF 78, which is detected by the inlet temperature sensor 80, are fed to the standard supply quantity determination section 86. From a map stored in advance, the standard supply quantity determination section 86 determines standard supply quantity Mb of the light oil required to raise the temperature of exhaust gas entering the DPF 78 to burn off particulate matter on the basis of the amount of accumulated particulate matter, which is estimated from a difference between the exhaust pressure Pin and the exhaust pressure Pout, and the exhaust temperature Tin.

The manner of determining the standard supply quantity Mb is not limited to this. There are a variety of known techniques that can be used.

As in the first embodiment, correction of the standard supply quantity Mb determined by the standard supply quantity determination section 86 and control of light oil injection from the light oil addition valve 32 are carried out according a flow chart including the same steps as Steps S12 to S22 of the flow chart of FIG. 3.

Specifically, the standard supply quantity Mb determined by the standard supply quantity determination section 86 is sent to the target supply quantity determination section 88. To the target supply quantity determination section 88, exhaust pressure Pex detected by the exhaust pressure sensor 30 and light oil temperature Tf detected by the light oil temperature sensor 36 are fed, and the target supply quantity determination section 88 corrects the standard supply quantity Mb on the basis of these exhaust pressure Pex and light oil temperature Tf.

The correction of the standard supply quantity Mb is performed in the same manner as in the first embodiment. In connection with the exhaust pressure Pex, a correction factor Rp corresponding to the detected exhaust pressure Pex is determined from a map stored in advance (Step S12 in FIG. 3), where the map is prepared such that the correction factor Rp becomes smaller as the exhaust pressure Pex increases, as shown in FIG. 4. By dividing the standard supply quantity Mb by the correction factor Rp, the target supply quantity determination section 88 corrects the standard supply quantity Mb to obtain pressure-corrected supply quantity Mp (Step S14 in FIG. 3).

By correcting the standard supply quantity Mb using the correction factor Rp in this manner, the pressure-corrected supply quantity Mp is greater than the standard supply quantity Mb when the exhaust pressure Pex is higher than the exhaust pressure value in the standard state. Thus, the shortage of supply quantity due to an increase in exhaust pressure Pex is compensated for. Conversely, the pressure-corrected supply quantity Mp is smaller than the standard supply quantity Mb when the exhaust pressure Pex is lower than the exhaust pressure value in the standard state. Thus, the excess of supply quantity due to a decrease in exhaust pressure Pex is obviated.

In connection with the light oil temperature Tf, a correction factor Rt corresponding to the detected light oil temperature Tf is determined from a map stored in advance (Step S16 in FIG. 3), where the map is prepared such that the correction factor Rt becomes greater as the light oil temperature Tf increases, as shown in FIG. 5. By dividing the pressure-corrected supply quantity Mp by the correction factor Rt, the target supply quantity determination section 88 corrects the pressure-corrected supply quantity Mp to obtain target supply quantity Mt (Step S18 in FIG. 3).

Here, the correction using the correction factor Rt is made to the pressure-corrected supply quantity Mp. However, as mentioned in respect of the first embodiment, since the pressure-corrected supply quantity Mp results from correcting the standard supply quantity Mb on the basis of the exhaust pressure Pex, the correction using the correction factor Rt is essentially made to the standard supply quantity Mb. Accordingly, by correcting the pressure-corrected supply quantity Mp, or essentially, the standard supply quantity Mb using the correction factor Rt in this manner, the target supply quantity Mp becomes smaller as the light oil temperature Tf increases. Thus, the excess of supply quantity due to a rise in light oil temperature Tf is obviated. Conversely, the target supply quantity Mt becomes greater as the light oil temperature Tf decreases. Thus, the shortage of supply quantity due to a drop in light oil temperature is compensated for.

In the flow chart of FIG. 3, as stated above, first at Steps S12 and S14, the pressure-corrected supply quantity Mp is obtained by correcting the standard supply quantity Mb on the basis of the exhaust pressure Pex, and then at Steps S16 and S18, the target supply quantity Mt is determined by correcting the pressure-corrected supply quantity Mp on the basis of the light oil temperature Tf. It is to be noted, however, that the order of the steps is not limited to this, like the first embodiment.

Alternatively, it can be arranged such that first, the correction factor Rp corresponding to the exhaust pressure Pex and the correction factor Rt corresponding to the light oil temperature Tf are determined from the respective maps, and then, the target supply quantity Mt is obtained by dividing the standard supply quantity Mb by the correction factors Rp and Rt, successively.

Further, although in the above-described case, correction is made by dividing the standard supply quantity Mb, the pressure-corrected supply quantity Mp or the temperature-corrected supply quantity by the correction factor Rp or the correction factor Rt, it can be arranged such that the reciprocal of each correction factor is obtained from a map stored in advance so that correction is made by multiplying the supply quantity by the reciprocal.

After the target supply quantity Mt of the light oil required to remove the particulate matter accumulated in the DPF 78 by burning is determined in this manner, the supply control section 90 determines, from a map stored in advance, valve open time of the light oil addition valve 32 required for the light oil addition valve 32 to inject light oil in the target supply quantity Mt (Step S20 in FIG. 3). As in the first embodiment, since the control on the light oil addition valve 32 is performed in control cycles of a predetermined period, the map is prepared to give the valve open time of the light oil addition valve 32 corresponding to the target supply quantity Mt, in the form of duty cycle Dt relative to the maximum valve open time in one control cycle, as shown in FIG. 6.

After determining the duty cycle Dt corresponding to the target supply quantity Mt from the map, the supply control section 90 drives the light oil addition valve 32 to open according to the determined duty cycle Dt (Step S22 in FIG. 3), so that light oil in a quantity equivalent to the target supply quantity Mt is injected from the light oil addition valve 32 into the exhaust pipe 16. Due to the heat of the exhaust gas, the light oil injected into the exhaust pipe 16 in this manner decomposes into HC, and oxidation reaction of the HC is promoted on the oxidation catalyst 76 so that the HC burns to raise the exhaust temperature. The exhaust gas raised in temperature by the combustion of HC flows through the DPF 78, and thereby the particulate matter accumulated in the DPF 78 is burned off so that the particulate trap capacity of the DPF 78 is restored.

It is to be noted that the exhaust pressure sensor 30 is disposed upstream of the exhaust throttle valve 28. Thus, even when the pressure in the exhaust pipe 16 varies due to the opening and closing of the exhaust throttle valve 28, light oil is always supplied properly in the amount required for burning-off of the particulate matter accumulated in the DPF 78, in spite of variations in pressure in the exhaust pipe 16, since, as mentioned above, the standard supply quantity Mb is corrected on the basis of the exhaust pressure Pex detected by this exhaust pressure sensor 30.

As described above, in the exhaust gas purification device according to the fourth embodiment of the present invention, the amount of supply of light oil required to raise the exhaust temperature to remove the particulate matter accumulated in the DPF 78 by burning to thereby maintain the particulate trap capacity of the DPF 78 is controlled properly, without being affected by variations in exhaust pressure and in light oil temperature. Thus, the exhaust gas purifying function can be stably maintained, and the emission of excess light oil into the atmosphere can be prevented.

It is to be noted that although in the above-described exhaust gas purification device according to the fourth embodiment, the target supply quantity Mt is determined by correcting the standard supply quantity Mb of the light oil required to maintain the particulate trap capacity of the DPF 78, on the basis of both the exhaust pressure Pex and the light oil temperature Tf, the correction may be made on the basis of either of them. The control accuracy in this case is lower, compared with when the correction is made on the basis of both the exhaust pressure Pex and the light oil temperature Tf, but higher, compared with the conventional exhaust gas purification device which takes account of neither of the exhaust pressure and the light oil temperature.

Further, although the above-described fourth embodiment of the present invention is an exhaust gas purification device applied to the diesel engine, it is not limited to the diesel engine but applicable to any types of engines designed to remove particulate matter from exhaust gas by means of a DPF.

In the above, embodiments of the present invention has been described. The present invention is, however, not limited to the above-described embodiments. When applied to any exhaust gas purification device designed to supply an auxiliary agent for maintaining the exhaust gas purifying function of an exhaust gas purification means, into an exhaust passage, upstream of the exhaust gas purification means, the present invention can produce similar positive effects. 

1. An exhaust gas purification device for an internal combustion engine, comprising: an exhaust gas purification means provided in an exhaust passage of the internal combustion engine for purifying exhaust gas exhausted from the internal combustion engine, an auxiliary agent supply means for supplying an auxiliary agent for maintaining the exhaust gas purifying function of the exhaust gas purification means, into the exhaust passage, upstream of the exhaust gas purification means, a variation factor parameter detection means for detecting the value of a variation factor parameter which causes variations in the amount of the auxiliary agent supplied, and a control means for controlling the auxiliary agent supply means, thereby regulating the amount of the auxiliary agent supplied into the exhaust passage, wherein the control means includes a standard supply quantity determination section for determining standard supply quantity of the auxiliary agent required to maintain the exhaust gas purifying function of the exhaust gas purification means; a target supply quantity determination section for determining target supply quantity of the auxiliary agent, by correcting the standard supply quantity determined by the standard supply quantity determination section, on the basis of the value of the parameter detected by the variation factor parameter detection means; and a supply control section for controlling the auxiliary agent supply means to supply the auxiliary agent in the target supply quantity determined by the target supply quantity determination section.
 2. The exhaust gas purification device for the internal combustion engine according to claim 1, wherein the variation factor parameter detection means includes an exhaust pressure detection means for detecting exhaust pressure in the exhaust passage, upstream of the exhaust gas purification means, and the variation factor parameter is the exhaust pressure.
 3. The exhaust gas purification device for the internal combustion engine according to claim 2, wherein the auxiliary agent supply means is designed to supply and stop supplying the auxiliary agent by opening and closing a solenoid valve, and the supply control section performs duty cycle control of opening and closing of the solenoid valve so as to supply the auxiliary agent into the exhaust passage in the target supply quantity.
 4. The exhaust gas purification device for the internal combustion engine according to claim 3, wherein the supply control section performs the duty cycle control of opening and closing of the solenoid valve such that the duty cycle becomes greater as the exhaust pressure increases.
 5. The exhaust gas purification device for the internal combustion engine according to claim 1, wherein the variation factor parameter detection means includes an auxiliary agent temperature detection means for detecting the temperature of the auxiliary agent, and the variation factor parameter is the auxiliary agent temperature.
 6. The exhaust gas purification device for the internal combustion engine according to claim 5, wherein the variation factor parameter detection means further includes an exhaust pressure detection means for detecting exhaust pressure in the exhaust passage, upstream of the exhaust gas purification means, and the target supply quantity determination section determines the target supply quantity by correcting the standard supply quantity on the basis of the exhaust pressure detected by the exhaust pressure detection means and the auxiliary agent temperature detected by the auxiliary agent temperature detection means.
 7. The exhaust gas purification device for the internal combustion engine according to claim 6, wherein the auxiliary agent supply means is designed to supply and stop supplying the auxiliary agent by opening and closing a solenoid valve, and the supply control section performs duty cycle control of opening and closing of the solenoid valve so as to supply the auxiliary agent into the exhaust passage in the target supply quantity.
 8. The exhaust gas purification device for the internal combustion engine according to claim 7, wherein the supply control section performs the duty cycle control of opening and closing of the solenoid valve such that a duty cycle for the duty cycle control becomes smaller as the auxiliary agent temperature increases.
 9. The exhaust gas purification device for the internal combustion engine according to claim 1, wherein the exhaust gas purification means is an NOx adsorption catalyst designed to adsorb NOx in exhaust gas that enters into the NOx adsorption catalyst when the air-fuel ratio of the exhaust gas is lean, and release and reduce the adsorbed NOx when the air-fuel ratio of the exhaust gas is rich, the auxiliary agent supply means supplies fuel as the auxiliary agent into the exhaust passage, upstream of the NOx adsorption catalyst, and the standard supply quantity determination means determines standard supply quantity of the fuel required to cause the NOx adsorption catalyst to release and reduce the NOx adsorbed by the NOx adsorption catalyst.
 10. The exhaust gas purification device for the internal combustion engine according to claim 1, wherein the exhaust gas purification means is an NOx adsorption catalyst designed to adsorb NOx in exhaust gas that enters into the NOx adsorption catalyst when the air-fuel ratio of the exhaust gas is lean, and release and reduce the adsorbed NOx when the air-fuel ratio of the exhaust gas is rich, the auxiliary agent supply means supplies fuel as the auxiliary agent into the exhaust passage, upstream of the NOx adsorption catalyst, and the standard supply quantity determination means determines standard supply quantity of the fuel required to restore the NOx adsorption capacity of the NOx adsorption catalyst that has lowered due to sulfur component adsorbed from the exhaust gas by the NOx adsorption catalyst, by causing the NOx adsorption catalyst to release the adsorbed sulfur component.
 11. The exhaust gas purification device for the internal combustion engine according to claim 1, wherein the exhaust gas purification means is a NOx catalyst designed to selectively reduce NOx in the exhaust gas, the auxiliary agent supply means supplies urea water as the auxiliary agent into the exhaust passage, upstream of the NOx catalyst, and the standard supply quantity determination means determines standard supply quantity of the urea water required for the NOx catalyst to selectively reduce the NOx in the exhaust gas.
 12. The exhaust gas purification device for the internal combustion engine according to claim 1, wherein the exhaust gas purification means is a particulate filter designed to trap particulate matter in the exhaust gas, the auxiliary agent supply means supplies fuel as the auxiliary agent into the exhaust passage, upstream of the particulate filter, and the standard supply quantity determination means determines standard supply quantity of the fuel required to regenerate the particulate filter by burning off particulate matter trapped by the particulate filter.
 13. The exhaust gas purification device for the internal combustion engine according to claim 2, further comprising an exhaust throttle disposed in the exhaust passage to regulate exhaust gas flow rate in the exhaust passage, wherein the exhaust pressure detection means detects the exhaust pressure in the exhaust passage, upstream of the exhaust throttle, and the auxiliary agent supply means is disposed upstream of the exhaust throttle.
 14. The exhaust gas purification device for the internal combustion engine according to claim 6, further comprising an exhaust throttle disposed in the exhaust passage to regulate exhaust gas flow rate in the exhaust passage, wherein the exhaust pressure detection means detects the exhaust pressure in the exhaust passage, upstream of the exhaust throttle, and the auxiliary agent supply means is disposed upstream of the exhaust throttle. 